Importancia

The effects of albedo enhancement is analysed on the radiation balance at solar noon (RB, Wm- 2_{), cloud coverage (CC, %), water mixing ratio (WMR, gwater/kgdry air), 2-m air temperature (T2,} o_{C), planetary boundary layer height (PBLH, m), ozone (O3, ppb), fine particulate matters (PM2.5,}

µg/m3_{) and nitrogen dioxide (NO2, ppb) concentrations. Table 8 presents the differences between}

CTRL and ALBEDO (CTRL−ALBEDO) scenarios in four cases (ISR-BASE, ISR-AR-DE, ISR-

AC-SDE, ISR-ARC-IDE) over the North, Center and South part of the Greater Montreal Area during the 2011 heat wave period.

20 25 30 35 40 0 12 0 12 0 12 T2 (oC) 0 20 40 60 80 100 0 12 0 12 0 12 RH (% ) 0 5 10 15 20 25 30 0 12 0 12 0 12 PM 2.5 (µg/m 3) 0 20 40 60 80 0 12 0 12 0 12 O3 (ppb)

Here, the effects of increasing solar reflectivity are presented in the ISR-BASE case simulation, where the radiation and cloud feedbacks are not considered, and convective parameterization is not activated. The simulation results show that albedo enhancement leads to a net decrease in daily 2-m air temperature by up to 0.7°C in the Center and 0.5°C in other parts of the domain during the 2011 heat wave period. The water mixing ratio reduces by 0.2g/kg across Montreal and cloud coverage indicates no change because of albedo increment. The heat island mitigation strategy causes a decline in solar noon radiative balance by almost 20Wm-2_{. The planetary boundary layer}

height lower by 28m in the Center and 20m in the North and South part of Montreal. Decreasing temperature leads to a decrease in planetary boundary layer height, which reduces the advection and diffusion of pollutants. Hence, this phenomenon increases the pollutant concentrations and also assists the O3 and NO reaction rates to produce NO2. This is the reason that the ozone

concentration is higher in some parts of the domain. On the other hand, by decreasing air temperature, the rate of temperature-sensitive photochemical reaction rates reduces and thus affords a decrease in daily ozoneconcentrations by nearly 4ppb in the Greater Montreal Area during the heat wave period. Albedo enhancement causes a decline in fine particulate matter by 4µg/m3 _{and minimal changes to nitrogen dioxide concentrations.}

7.7.2. Effects of ISR on Aerosol, Radiation and Cloud Interactions in the Urban Atmosphere

The effects of albedo enhancement on aerosol and radiation interactions show a slight increase in air temperature by ~ 0.2o_{C in the Center area and a decrease by the same amount in the other}

parts of the domain. The reason for these changes is because of the simulation configuration that only the radiation feedback is considered, and the convective parametrization and cloud formation has not been activated. Thus, the results indicate that because of the absorbent components in the Center part of the GMA, although the albedo is increased, but the outgoing longwave radiation from the surface is trapped by atmospheric aerosols. Therefore, without considering the convective parametrizations, the air temperature increases and heats the local atmosphere. An increase in temperature lead to a rise in water mixing ratio by nearly 0.2g/kg in the Center and a decline by the same amount in other part of the domain, but no changes in cloud coverage. Increasing surface reflectivity causes a decrease in radiative balance at solar noon by around 15Wm-2 _{across Montreal.}

Heat island mitigation strategy reduces the planetary boundary layer height across the domain by 10m. Surface albedo modifications cause a decrease in the temperature-dependent photochemical reaction rates in the atmosphere, even though it is minimal. The O3 (ppb), PM2.5 (µg/m3), and NO2

(ppb) concentrations decrease slightly as a consequence of increasing surface reflectivity on aerosol and radiation interactions.

The aerosol and cloud interactions show that albedo enhancement leads to a slight decrease in 2-m air temperature. This occurs because the aerosol-radiation interactions have not been estimated in these simulations. As temperature reduces, the evaporation from water bodies reduces, and thus a decrease in water mixing ratio is expected. But the results show that water mixing ratio behaves differently and increases slightly across the domain. The cloud coverage also rises by 3% across the entire domain. Increasing solar reflectance causes a decrease in radiative balance at solar noon by around 20Wm-2_{. Albedo enhancement causes a decrease in PBLH in a}

range of 20m in the GMA. The fine particulate matters and ozone concentrations decrease by 1µg/m3 _{and 1ppb in aerosol-cloud (ISR-AC-SDE) simulation, respectively.}

Considering the nonlinear and complex interaction of aerosol-radiation-cloud in the atmosphere, the 2-m air temperature decreases by 0.5o_{C in the Center and North parts of the}

domain and 0.3o_{C in the Southern area. The water mixing ratio decreases to 0.5 g/kg in the Center}

and 0.3g/kg in the North and South regions. The cloud coverage declined by 3-5% across the Greater Montreal Area. Albedo enhancement leads to a net decrease in radiative balance at solar noon by 25Wm-2 _{in the Center and 22Wm}-2 _{in the Northern and Southern regions. Increasing solar}

reflectivity imposes a decrease in planetary boundary layer height to 25m and 20m in the Center and other parts of Montreal, respectively. Heat island mitigation strategy affords a decrease in temperature and thus ozone concentrations to almost 3ppb across the entire domain. The fine particulate matter also reduces to about 3µg/m3 _{in the Center and 2µg/m}3 _{in other areas during the}

2011 heat wave period. The NO2 concentrations reduces slightly compared to PM2.5 and O3

concentrations across the domain of interest.

Table 8. The differences between CTRL and ALBEDO scenarios of T2 (o_{C), RH2(%), O3 (ppb), PM2.5 (µg/m}3_{), NO2 (ppb), NO} (ppb) over North, Center and South part of GMA during the 2011 heat wave period

CTRL−ALBEDO Region ISR-BASE ISR-AR-DE ISR-AC-SDE ISR-ARC-IDE

Δ RB at noon (Wm-2₎ North _Center 18 ₂₀ 15 ₁₆ 20 ₂₁ 22 ₂₅

South 17 17 21 23

Δ Cloud coverage (%) North Center No change No change No change No change 3 3 3 5

South No change No change 3 3

ΔWMR (g/kg) North Center 0.2 0.2 -0.2 0.2 -0.2 -0.2 0.3 0.5

South 0.2 0.2 -0.2 0.3

South 0.54 0.21 0.25 0.51 Δ PBLH(m) North Center 22 28 10 8 20 23 22 25 South 20 10 20 18 24-h avg. O3(ppb) North 3.67 0.59 0.74 2.66 Center 4.41 0.56 0.68 2.76 South 3.55 0.51 0.57 2.09 24-h avg. PM2.5(µg/m3) North 3.11 0.98 1.03 2.88 Center 3.67 0.78 0.54 2.59 South 3.21 0.60 0.67 1.91 24-h avg. NO2(ppb) North 0.19 0.27 0.25 0.28 Center 0.36 0.18 0.31 0.35 South 0.13 0.16 0.24 0.23

7.8. Discussion and Limitations of Aerosol, Radiation and Cloud Interactions